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Observational data on companion statistics around young stellar systems isneeded to flesh out the formation pathways for extrasolar planets and browndwarfs. Aperture masking is a new technique that is able to address animportant part of this discovery space. We observed the two debris disk systemsHD 92945 and HD 141569 with sparse aperture masking (SAM), a new mode offeredon the NaCo instrument at the VLT. A search for faint companions was performedusing a detection strategy based on the analysis of closure phases recoveredfrom interferograms recorded on the Conica camera. Our results demonstrate thatSAM is a very competitive mode in the field of companion detection. We obtained5 sigma high-contrast detection limits at lambda/D of 2.5x10^{-3} (\Delta L' =6.5) for HD 92945 and 4.6x10^{-3} (\Delta L' = 5.8) for HD 141569. According tobrown dwarf evolutionary models, our data impose an upper mass boundary for anycompanion for the two stars to, respectively, 18 and 22 Jupiter masses atminimum separations of 1.5 and 7 AU. The detection limits is mostly independentof angular separation, until reaching the diffraction limit of the telescope.We have placed upper limits on the existence of companions to our targetsystems that fall close to the planetary mass regime. This demonstrates thepotential for SAM mode to contribute to studies of faint companions. Wefurthermore show that the final dynamic range obtained is directly proportionalto the error on the closure phase measurement. At the present performancelevels of 0.28 degree closure phase error, SAM is among the most competitivetechniques for recovering companions at scales of one to several times thediffraction limit of the telescope. Further improvements to the detectionthreshold can be expected with more accurate phase calibration.

Sparse aperture masking at the VLT I. Faint companion detection limits for the two debris disk stars HD 92945 and HD 141569